1. Field of the Invention
The present invention relates to placing flat work in electric contact when carrying out electrolytic processes in continuous processing plants, preferably for processing printed circuit boards and printed circuit films.
2. Description of Related Art
Electrolytic methods are used for manufacturing printed circuit boards. Metal, mostly copper, is more specifically deposited to form strip conductors and soldering and bonding places. Generally, the starting material consists of dielectric boards or films that are coated with copper layers on either side. After through bores have been made through the material, a layer of copper, of, e.g. 30 μm thick, is applied all over the surfaces, a layer of copper being deposited on the walls of the bores as well. In order to be able to strip conductors delineations and other metal structures, in a variant of the method, a structured mask is first applied to the surfaces of the boards or films resp., those regions of the surface remaining bare in which the structures are to be formed. For this purpose, the conductive pattern is printed in negative. This is performed by means of screen-printing and, with fine-line printed circuits, by means of reproduction techniques (photoresist). A layer of photoresist usually has a thickness of from 25 μm to 40 μm and constitutes a high-quality electric insulator. No metal is to be deposited onto the resist during patterning. Subsequently, metal is deposited in those regions of the covering mask that had been kept bare and the metal covered by the covering mask is finally removed by etching once the mask has been taken off. This method is referred to as pattern plating process.
In the electrolytic process, a conductive surface to be electrolytically processed must be electrically connected to the power source required for this purpose. On electroplating it is the negative pole that has to contact this surface, on etching, it is the positive pole, the respective opposite pole being electrically connected to a counter electrode which contacts a processing fluid as does the surface to be processed. In continuous processing plants of the printed circuit board technique, contact is made among others with rolls. The work, the printed circuit boards, is for example conveyed in horizontal direction through the various wet-chemical processing and rinsing stations by means of lower and upper driven rolls or wheels. The rolls are usually made from a material that resists the electrolyte used, such as for example PP (polypropylene), PE (polyethylene) or PVC (polyvinyl chloride). Rolls provided with a sponge coating whose pores are either open or closed are used for squeezing fluids out and for simultaneous transportation.
For placing the surfaces of the printed circuit boards in electric contact in electrolytic processing plants, pairs of metallic contacting rolls that is, upper and lower rolls are employed. The current from the power source of the bath is delivered to said rolls by way of a sliding contact for example. The metallic surface of each roll unreels on the surface of the boards to be processed. These rolls serve to convey the work and concurrently to transmit power to its surface.
Such a device for electroplating printed circuit boards is described in DE 36 03 856 A1. Inside the electroplating plant, a pair of rotary rolls serving as a cathode take hold of and convey the printed circuit boards. The rolls consist of stainless special steel or titanium.
A similar device is described in EP 0 959 153 A2: metal rolls are arranged on either side of a horizontal conveying plane. The printed circuit boards to be processed are placed in electric contact by way of these rolls. For this purpose, an electrical connection is made between an external power source and the metal rolls via a rotating current transmission device.
The document JP-A-63-297588, Patent Abstracts of Japan, discloses an electrolytic method of processing electrically isolated circuit structures on printed circuit boards. For electroplating, the sometimes very small isolated areas are placed in electric contact by means of a rotating metallic brush.
The known devices and methods do not permit to deposit metal layers of uniform thickness without damaging the printed circuit boards. For it has been found that variations occur in the thickness of the coatings in particular when large sized boards are being electrolytically processed and that the boards may even be damaged in the course of processing. It has also been found that the coating thickness of the individual strip conductors can also vary within a wide range when electroplating conductors patterns during the pattern-plating-process. The known documents do not discuss why these problems occur and how to eliminate them.